This invention relates to, in general, a method of bonding semiconductor wafers, and more particularly, a method of bonding edge rounded semiconductor wafers of the same size.
Typically, semiconductor wafers are provided having the edges rounded to prevent "crowning" when an epitaxial layer is grown on the surface of the substrate. These wafers are typically 500 microns thick. In this technology, the two wafers, an active wafer on which semiconductor devices will be fabricated and a handle wafer to provide support, are bonded to provide a means for isolation or for bonding two dissimilar materials.
After bonding the exposed surface of active wafer is polished back to a thickness of 2-40 microns. Because of the edges of the wafer were rounded, this polishing step left remaining a small portion of the active wafer at the perimeter which was not in physical contact with the handle wafer. This portion would fracture later in the process or in the polishing step, causing polish scratches or contamination.
Prior art methods have been used to solve the problem stated above. One such method involves using photolithography and etch techniques to remove a perimeter portion of the active wafer after bonding. However, this technique is costly and adds cycle time to the process. Another method involves reducing the diameter of the active wafer before bonding to the handle wafer. The disadvantage of this method is that bonding of two different size wafers is more difficult. In addition, it is believed that the problem would not be solved, because the reduced diameter wafer would be preferably polished after the diameter is reduced to relieve stress in the wafer. The polishing creates a rounded surface at the edges that will not be in physical contact with the handle wafer after bonding, because the bonding surface of the handle wafer will be level, but the bonding surface of the active wafer will not be level. If the polishing step is not performed after reduction of the diameter, then defects will be created later in the process.
As can be seen, it would be desirable to have a method of bonding two wafers without the chipping of the edges of the active wafer. This method should desirably be inexpensive and easier to perform than the methods that are presently available.